PROBOSCIS OF PEDICULUS 535 



anterior and the other posterior to the dilator fibres, with which they 

 intermingle to a certain extent. They are attached to the angles of 

 junction of the plates which make up the wall of the pharynx. The 

 posterior division of the sphincter is continued into the wall of the 

 oesophagus, and is gradually reduced in size as it passes backwards, 

 disappearing entirely before the oesophagus enters the substance of the 

 brain. The arrangement of these sphincter muscles makes it possible for 

 the pharynx to be shut off from the buccal cavity in front and from the 

 oesophagus behind. 



In addition to the muscles which regulate the size of the pharynx 

 there is another pair which passes to the chamber from the posterior part 

 of the head. These arise from the lateral wall of the head cavity 

 posterior to the brain, and pass inwards and forwards, converging towards 

 the oesophagus, with which they pass between the supra- and infra- 

 oesophageal ganglia, the substance of the muscle being replaced by 

 tendon as it passes forwards. These tendons spread out to gain inser- 

 tion over a considerable area of the wall of the pharynx, and probably 

 also pass forwards to reach the tube which connects it with the buccal 

 cavity. Their position suggests that they represent the retractors of 

 the oesophagus in Tabanus (Plate V, fig. 1), and that their function is 

 to retract the buccal cavity at the end of the act of sucking, by reducing 

 the membraneous portion of the canal to the loop which it forms in the 

 position of rest. 



In the region of the looped portion of the canal there are several 

 strong bands of muscle which pass obliquely from the middle line 

 dorsally to the lateral walls of the head, in the region of the antennae, 

 extending as far as the origins of the dilator muscles of the pharynx. 

 These are not attached to the stomodaeum at any point, so that their 

 only action can be to approximate the dorsal and lateral walls of the 

 head, and thereby to reduce its transverse diameter. The most probable 

 function for these muscles would appear to be the increase of the intra- 

 cranial pressure ; an alternation of contraction between these muscles 

 and the dilator muscles of the pharynx and buccal cavity would mate- 

 rially assist in the propulsion of the blood from these chambers into the 

 oesophagus. 



The proboscis of the louse is enclosed, in the position of rest, in a 

 membraneous sheath, which, as already stated, extends from the level of 

 the mouth opening to the posterior end of the head, ^ Ppobogcjg and 

 and represents an imagination of the integument. In jtg 8heath 

 most sections this sheath appears as very much wider 



